Gas measurement device and measurement method thereof

ABSTRACT

A gas measurement device measures gas using a gas sensor including a sense resistance exposed to the gas and a reference resistance not exposed to the gas. The gas measurement device applies a first current value and a second current value to the sensor. A detector functions to detect a first resistance variation and a second resistance variation of the sense resistance exposed to the gas with respect to the reference resistance as a function of the first current value and the second current value, respectively. The resistance variation dependent on relative humidity is then determined as a function of the first and second resistance variations and a first constant. The resistance variation dependent on gas content is then determined as a function of the first and second resistance variations and a second (different) constant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application from U.S. application forpatent Ser. No. 14/726,823 filed Jun. 1, 2015, which claims priorityfrom Italian Application for Patent No. MI2014A001197 filed Jul. 2,2014, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a gas measurement device andmeasurement method.

BACKGROUND

A thermal conductivity detector (TCD) is well known in the state of theart. This is an environmental sensor device widely used for themeasurement of the amount of gas in the environment. The operation isbased on the fact that each gas has an inherent thermal conductivity anda filament (thermal resistor) changes its temperature as a function ofthe amount of gas that surrounds it. The most appropriate sensingelement shape is that of a suspended thin finger, for which thetemperature of the central part can locally reach even values of severalhundred degrees. The feature that the finger is totally suspended allowsfor enhancing the amount of heat exchange with the gas in which it isimmersed. The warming effect of the suspended finger is induced throughan electrical stress of the sensor, that is by the flow of the currentthrough the finger. The sensor is able to better discriminate the gaseswhose conductivity is much different than normal air (roughly nitrogenN₂ (79%), oxygen O₂ (19%), carbon dioxide CO₂ (0.04%), plus other gaseswith negligible quantities: for example the carbon oxide CO is a fewppm).

When a current flows through the finger, the value of the resistance ofthe finger changes. The measurement of the resistance value allows formeasuring the conductivity of the gas mixture which depends of the molarfraction of the gas of interest.

However, it is difficult in principle to discriminate which gas ismainly responsible for the conductivity variation of the mixture of gas.For example, carbon dioxide CO₂ has a lower thermal conductivity thandry air, therefore if its percentage increases inside the mixture, thiswill raise the temperature of the sensor with a consequent increase ofthe value of the measured resistance.

The TCD sensor operates in accordance with the thermodynamic equilibriumamong heat generated by the current flow, heat exchange with thematerial of which the sensor is made (e.g. polysilicon crystalline), andheat exchange with the gas mixture surrounding it. The ambienttemperature determines the equilibrium value of the sensor in standarddry air. To take into account and compensate the variation of ambienttemperature a Wheatstone bridge as the sensor structure could be used.The reference branches of the bridge are of the same nature andpositioned in the vicinity of the sensor so as to be sensitive to thesame way to changes in ambient temperature, with the difference thatthese branches will not be exposed to the mixture of gas as is thesensor.

The Relative Humidity (RH) is the amount of water vapor (gas) present inthe environment compared to a saturated environment in the sameconditions of pressure and temperature. The thermal conductivity ofwater vapor is much larger than the dry air therefore an increase inrelative humidity produces a lowering of the temperature of the sensorwith a consequent reduction of the value of the measured resistance. Thecontribution of the RH value of the measured resistance could be 1/10compared to the change of resistance in the presence of carbon dioxideCO₂, therefore, this is a parameter to measure and correct. Typicallythe correction is made by means of a dedicated sensor for themeasurement of the RH.

SUMMARY

One aspect of the present disclosure is to provide a gas measurementdevice of simple architecture.

One aspect of the present disclosure is a gas measurement device formeasuring gas by means of a gas sensor comprising at least oneresistance exposed to at least one gas and at least one referenceresistance not exposed to the gas, said gas measurement devicecomprising: a control device configured to manage the gas sensor so thatthe gas sensor receives at least a first current value and a secondcurrent value, a detector to detecting a first resistance variation anda second resistance variation of the resistance exposed to the gas withrespect to the reference resistance as a function of the first currentvalue and the second current value respectively, and a calculationcircuit configured to calculate at least a first and a second equationswherein the first equation is given by the difference between the firstresistance variation multiplied by a first constant and the secondresistance variation while the second equation is given by thedifference between the first resistance variation multiplied by a secondconstant and the second resistance variation, the first constant and thesecond constant having different values.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, a preferredembodiment thereof is now described, purely by way of non-limitingexample and with reference to the annexed drawings, wherein:

FIG. 1 shows a block diagram of a measurement apparatus comprising a gassensor device and a gas measurement device according to the presentdisclosure;

FIG. 2 shows a more detailed block diagram of the gas measurement deviceaccording to the present disclosure;

FIGS. 3-5 show the waveforms of the resistance variations ΔR(Il), ΔR(Ih)as a function of the concentration of carbon dioxide CO₂ and thewaveform of a resistance value as a function of the relative humidityRH;

FIG. 6 shows the waveforms of the resistance variation value as afunction of the different concentrations of CO₂.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a measurement apparatus comprising a gassensor device 1, that is a TCD sensor, and a gas measurement device 100according to the present disclosure.

The gas sensor device 1 comprises at least one variable resistance R2exposed to the gas and a reference resistance R1 which is not exposed tothe gas; the reference resistor R1 has the value of the resistance R2 atthe condition of dry air and room temperature. The value of theresistance R2 varies when exposed to the gas, the humidity and thetemperature. Preferably, the gas sensor device 1 is a Wheatstone bridgeincluding a couple of reference resistors R1 and a couple of resistorsR2 exposed to the gas; the use of a Wheatstone bridge allows forminimizing the dependence on the ambient temperature. The fourconnecting nodes A-D of the terminals of the resistances R1 and R2 ofthe Wheatstone bridge 1 are connectible respectively with a variablecurrent generator 200, to ground GND and to the gas measurement device100 able to receive the voltage signal at the output of the Wheatstonebridge 1.

The measurement device 100 (FIG. 1) comprises preferably a temperaturesensor 102 in the case wherein it is necessary to provide a temperaturecompensation of the signals at the output of the sensor device 1. Themeasurement device 100 comprises preferably a multiplexer 101,configured to receive the output signal of the sensor device 1 or theoutput signal of the temperature sensor 102. The measurement device 100comprises a device 103 configured to amplify the signal at the output ofthe multiplexer, an analog-to-digital converter 104 for converting theanalog signals at the input into a digital signal at the output, adigital controller 105 for processing the signal deriving from thesensor device and an interface 106 for outputting the processed signalto the outside.

The measurement device 100 is shown in more detail in FIG. 2. The device103 is preferably a low noise analog front end comprising the cascade oftwo fully differential switched-capacitor amplifiers configured toamplify the signal at the output of the multiplexer 101 and tocompensate the offset of the gas sensor 1 or the temperature sensor 102.The low noise analog front end 103 makes use of both chopping andcorrelated double sampling techniques, which ensure offset canceling andlow frequency noise filtering.

A managing device 109 manages the devices 101-106 and the variablecurrent generator 210; the managing device 109 manages the timing of thelow noise analog front end 103, the analog-to-digital converter 104 andthe digital controller 105. The managing device comprises a clockgenerator 111 configured to send two different clock signals atdifferent frequency, for example 1 Mhz and 40 Khz, to a phase generator110 which receives the output of the bit register 112.

When a gas having a concentration m is inside the gas sensor 1 at arelative humidity n, the managing device 109 is configured to effectuatethe following steps:

-   -   managing the variable current generator 210 to send a first        current value Il to the gas sensor 1 and detect the resistance        variation ΔR(Il) of the resistances R2 with respect to the        reference resistances R1;    -   managing the variable current generator 210 to send a second        current value Ih to the gas sensor 1 and detect the resistance        variation ΔR(Ih) of the resistances R2 with respect to the        reference resistances R1;    -   managing the digital controller 105 to calculate the resistance        variation Δh depending only on the relative humidity variation        by means of the following equation Δh=K1×ΔR(Il)−ΔR(Ih) and the        resistance variation Δc depending only on the gas concentration        variation by means of the following equation Δc=K2×ΔR(Il)−ΔR(Ih)        wherein K1 and K2 are constants having different values. In this        way the calculation of the above equations allow obtaining the        indirect measure of the relative humidity alone, independently        from the gas concentration, and of the gas concentration alone,        independently from the relative humidity, and    -   managing the interface 106 to output the resistance variations        Δh and Δc.

In the case wherein the concentrations of a first and a second gasesneed to be measured, the digital controller 105 is configured to:

-   -   manage the variable current generator 210 to send a first        current value Il to the gas sensor 1 and detect the resistance        variation ΔR(Il) of the resistances R2 with respect to the        reference resistances R1;    -   manage the variable current generator 210 to send a second        current value Ih to the gas sensor 1 and detect the resistance        variation ΔR(Ih) of the resistances R2 with respect to the        reference resistances R1;    -   manage the digital controller 105 to calculate the resistance        variation Δc1 depending only on the concentration variation of        the first gas by means of the following equation        Δc1=K21×ΔR(Il)−ΔR(Ih)) and the resistance variation Δc2        depending only on the concentration variation of the second gas        by means of the following equation Δc2=K22×ΔR(Il)−ΔR(Ih),        wherein K21 and K22 are constants having different values. In        this way the calculation of the above equations allow obtaining        the indirect measure of the concentration of the first gas        independently from the concentration of the second gas and vice        versa, and    -   manage the interface 106 to output the resistance variations Δc1        and Δc2.

In FIGS. 3-5 the waveforms of the resistance variations ΔR(Il), ΔR(Ih)are shown wherein on the X axis the variation of the concentration ofthe gas is indicated while on the Y-axis the variation of the sensorresistance is indicated at the condition for a relative humidity RH=0,RH=30% and RH=60%. The further waveform is the resistance valueΔh=K1×ΔR(Il)−ΔR(Ih) which is independent on the variation of theconcentration of the gas and depends only on the relative humidity RH.

FIG. 6 show the resistance values Δc=K2×ΔR(Il)−ΔR(Ih) for gasconcentrations m=0%, m=10% and m=20% which depend only on theconcentration variation of the carbon dioxide CO₂ and are independent onthe relative humidity RH.

Preferably the constants K1 and K2 have respectively the values of 1.827and 2.165. A method for calculating the appropriate value of theconstants K1 and K2 is now described.

The thermal conductivity of a gas mixture depends on the molar fractionof the gases of the mixture, on the conductivity of the gases and on thedynamic viscosity according to the Chapman-Enskog model.

In first approximation, starting from the Chapman-Enskog model (“Themathematical theory of non-uniform gases: an account of kinetic theoryof viscosity, thermal conduction and diffusion in gases” S. Chapman, TG.Cowling 1970, incorporated by reference) and obtaining a linearequation, the thermal conductivity of a gas mixture is linearlyproportional to the temperature and the concentration of gases of themixture.

The resistance variation ΔR (that is the variation of the resistance R2with respect to the reference resistance R1) is a linear function ofboth the concentration of the matters to be examined (the concentrationof gas and the humidity or the concentrations of two gases) and thecurrent flowing through the resistance R2, preferably, in the casewherein the sensor is a Wheatstone bridge, the resistance variation ΔRis a linear function of both the concentration of the matters to beexamined and the current flowing through the bridge 1.

In fact, balancing and solving the equation for the thermoelectricequilibrium of the system comprising the bridge 1 and the gas mixture,the resulting temperature at the equilibrium is approximately a linearfunction of the concentrations of gas and humidity and of the currentflowing through the bridge 1.

At the thermoelectric equilibrium it is necessary to consider the powerdissipated by Joule effect on the resistance R2, P=R×I² wherein I is thecurrent flowing through the bridge 1, and the amount of the heatexchange due to the thermal conductivity of the gas mixture,

$\frac{Q}{T} = {K \times \frac{A}{dx}\Delta\; T}$where A is the surface of the resistance R2, dx is the thickness of theresistance R2 and ΔT is the temperature variation; at the thermoelectricequilibrium it is obtained that the temperature variation ΔT is a linearfunction of the concentrations of gas and humidity and of the currentflowing through the bridge 1 The resistance variation AR depends on thetemperature variation ΔT according to the

ΔR=R0×(1+αΔT) where α is the thermal coefficient of the resistance anddepends on the material of the resistive bridge and R0 is the resistancevalue at room temperature, therefore even the resistance variation ΔR,so as the temperature variation ΔT, is a linear function of theconcentrations of gas and humidity and of the current flowing throughthe bridge 1. The resistance variation ΔR as linear function of theconcentrations of gas and humidity and of the current flowing throughthe bridge 1 can be represented by the following equationΔR=(a×I+b)×m+(c×I+d)×n wherein m is the concentration of gas, n is theconcentration of humidity, I is the current flowing through the bridge 1and a, b, c and d are parameters depending on the balance of the systemwhich are determined by effectuating four calibration measurements withknown gas and humidity concentrations and currents.

After determining the parameters a, b, c and d two measurements of theunknown mixture are effectuated with the unknown concentrations m and nand two different current values Il and Ih; solving said two equationsand calculating the resistance variation as function of the current,that is ΔR(Il) and ΔR(Ih), the unknown values of the concentrations mand n are obtained.

Considering the generic equation ΔR=K×ΔR(Il)−AR(Ih), exist only twovalues K1 and K2 of K which allow the m and n concentration componentsto become null. The equation becomes:ΔR(K)=(a×(K×Il−Ih)+b×(K−1))×m+(c×(K×Il−Ih)+d×(K−1))×n and setting equalto zero the m and n concentration components the values

${K\; 1} = {{\frac{{a \times {Ih}} + b}{{a \times {Il}} + b}\mspace{14mu}{and}\mspace{14mu} K\; 2} = \frac{{c \times {Ih}} + d}{{c \times {Il}} + d}}$are obtained. In this way each one of the results Δc(K2) and Δh(K1)depends on the concentration variations only of one of two unknownconcentrations.

The invention claimed is:
 1. An apparatus for gas measurement,comprising: a gas sensor comprising a sensing resistance exposed to afirst gas and a second gas and at least one reference resistance notexposed to the first and second gases, a current supply circuitconfigured to apply to the gas sensor a current having at least a firstcurrent value Il and a second current value Ih, a detector circuitconfigured to detect a first resistance variation ΔR(Il) of said sensingresistance exposed to the first and second gases with respect to thereference resistance as a function of the first current value Il anddetect a second resistance variation ΔR(Ih) of the same sensingresistance exposed to the first and second gases with respect to thesame reference resistance as a function of the second current value Ih,and a calculation circuit configured to calculate: a first resistancevariation dependent on gas concentration of the first gas as a functionof the first resistance variation ΔR(Il) and the second resistancevariation ΔR(Ih) and a first constant K21 having a value by which thefirst resistance variation calculation is independent of concentrationvariation of the second gas; and a second resistance variation dependenton gas concentration of the second gas as a function of the firstresistance variation ΔR(Il) and the second resistance variation ΔR(Ih)and a second constant K22 having a value by which the second resistancevariation calculation is independent of concentration variation of thefirst gas; wherein the first constant K21 and the second constant K22have different values.
 2. The apparatus according to claim 1: whereinthe first resistance variation is calculated as a function of adifference between the first resistance variation ΔR(Il) multiplied bythe first constant K21 and the second resistance variation ΔR(Ih); andwherein the second resistance variation is calculated as a function of adifference between the first resistance variation ΔR(Il) multiplied bythe second constant K22 and the second resistance variation ΔR(Ih). 3.The apparatus according to claim 2, wherein the calculation of the firstresistance variation utilizes the following equation K21×ΔR(Il)−ΔR(Ih)and the calculation of the second resistance variation utilizes thefollowing equation K22×ΔR(Il) −ΔR(Ih).